I don't often get visitors in the cockpit. A lot of this is probably because I fly "one of those itty-bitty planes," and there's not a whole lot of room up front. However, I really enjoy when somebody takes an interest in what's going on up front, especially when it's a younger kid. With the older people it shows that they have an interest in understanding what we're doing and what we have to deal with; when children visit it's great to see an unadulterated appreciation for the wonder of flight.
My favorite sticker on my chart case is a sticker that a kid made for me on a flight from LGA to SAV; I'm trying to figure out how I'm going to get it on my next chart case when I have to replace this one.